This invention relates to a method and apparatus for measuring a chemical characteristic of a liquid. More particularly, it relates to a method and apparatus for measuring the pH value of a blood sample.
The pH value of blood is a frequently measured physiological parameter which provides an indication of proper acid-base balance and blood-gas exchange through the lungs. In the field of obstetrics, such a test is often made if there are indications of fetal distress during labor which may be caused by the fetus not receiving sufficient oxygen from the mother through the placenta. In such cases, a blood sample is taken by making a small incision on the fetal scalp and placing a capillary tube in the proximity of the incision whereby the blood is drawn up into the interior portions of the tube through capillary action. The standard clinical device for measuring blood pH is the blood-gas analyzer such as the pH Blood Gas System and Supply manufactured by Corning Medical Corporation. Such analyzers require the transfer of the blood sample from the capillary tube to a receptacle in the instrument. As is known in the art, if any ambient air mixes with the blood sample, the pH value will change. Hence, there is a good possibility of contaminating the blood sample during the transfer from the original collecting device to the instrument. Moreover, since such analyzers are complex and costly devices, they are typically located only in the hospital lab where they need to be operated by a skilled technician. As a result, there is often considerable delay between the time of taking the blood sample until the results from the lab are received. Of course, such delays are undesirable in emergency situations.
U.S. Pat. Nos. 3,911,901 to Niedrach et al, 3,049,118 to Arthur et al., and 3,399,667 to Nishimoto et al disclose representative devices for measuring the pH value of blood samples. However, they have all been relatively complex and costly to manufacture. Moreover, their use has been limited to trained personnel. Furthermore, none of them have permitted direct pH measurement from the same device which collected the blood sample. Another drawback in many of the prior art devices is that they required large quantities of the liquid to be tested. Unfortunately, it is often difficult to readily obtain sizable samples.